What Is Underground Fire?
An underground fire has three stages in its development:
- developed fire;
Inflammation is characterized by an increase in the amount of combustion material per unit of time, oxygen consumption for combustion, an increase in the concentration of carbon-containing gases in products of combustion, an increase in the temperature of products of combustion.
The developed subterranean fires are characterized by the full oxygen consumption for combustion and the maximum concentration of carbon-containing gases at a constant air consumption, combustion in a unit of time of a constant (maximum) amount of combustible material, and a constant temperature of products of combustion.
In the attenuation phase, there is an increase in oxygen concentration in combustion products, a decrease in the content of carbon-containing gases, and a decrease in the temperature of fire gases.
The development of underground coal fires depends on the power and duration of the initial thermal impulse, the amount and nature of the location of combustible material, and the speed of airflow in the hearth.
Development of a fire in an output fixed with wooden support can be presented as follows. At first, the flame spreads through the tightening in the direction of air movement and upwards. At the low velocity of the ventilation jet, the ignition is faster than at high velocity. In the fixed space, where the velocity of airflow is minimal, the flame spreads vertically and reaching the roof, covers the entire surface of the production.
Approximately, 10 minutes after the start of burning, puffs fall from the roof, forming on the soil pockets, which ignite other elements. Most of the perimeter of the mine is covered by fire.
Increasing the speed of the ventilation jet at this time contributes to more active fire development. In 35 -50 minutes after the beginning of the fire, puffs are completely burned. After 2 – 2.5 hours from the beginning of the fire burning underground, wooden support almost completely burns.
As the area of combustion increases, there is an increase in the temperature of combustion products, increasing the content of carbon monoxide and dioxide, methane, and hydrogen. When the temperature of fire gases 500 – 550 °C, the fire stabilizes. In this case, the concentration of oxygen in the products of combustion, as a rule, does not exceed 15 – 16%, while the carbon dioxide content reaches 5 – 6%.
Extinguishing Underground Fires Is Carried Out in the Following Ways:
- Active – direct influence on the fireplace by fire extinguishing means (water, chemical and air-mechanical foam, fire extinguishing powder, sand) or disassembling the fireplaces by pouring the burning mass of water. It is usually used in all open fires at the beginning of their occurrence;
- Passive – isolation of fire area by lintels with backfill if necessary, and tamponing of cracks in the heaps and containing rocks. Isolation shall be used when a fire cannot be eliminated directly, extinguishing due to the inaccessibility of combustion sites for direct exposure to fire extinguishing agents;
- Combined – direct extinguishing in combination with the insulation of fire areas, flooding with water, or filling with inert gases. It is used when the fire has taken a large size and direct extinguishing does not give the proper effect or when it is impossible to eliminate the fire only by isolation.
Active fire extinguishing is usually carried out by a fresh jet of air. At the same time, measures are taken to prevent the spread of fire in the outgoing stream (and into the adjacent workings) using water curtains, removal of wooden fasteners and other materials in a certain area, the installation of temporary fire-resistant jumpers, etc. are taken.